Electron beam microscopy involves generation of an electron beam, directing the electron beam at a specimen and detecting a result of the interaction of the electron beam with the specimen. The result of the interaction is typically secondary electrons and backscattered primary electrons, but may also be visible light, ultraviolet light, X-rays or the like. The electron beam source, the specimen and the detector are located within a vacuum enclosure. Traditionally, electron beam microscopy has been performed using single beam or projection techniques.
Single, or point, beam systems, such as the scanning electron microscope (SEM) are used extensively for microscopy due the relative simplicity of design and use. In point beam systems, an electron beam is focused to a small diameter, and particles emitted from the specimen are detected as a function of position to generate an image of the specimen. The diameter of the electron beam sets a lower limit on resolution. For many applications using this approach, the primary limitation on performance is space charge interaction within the beam. This causes irreversible defocusing of the beam and thus limits the current that can be delivered to the specimen for a given resolution. The deliverable current determines the speed with which the microscope can scan a given specimen with a given signal-to-noise ratio. For many applications, higher speed is desirable.
In projection techniques, areas of the specimen larger than a single pixel are exposed to a uniform flood beam of electrons, and a result of the interaction with the specimen is detected with an imaging detector to generate an image of the specimen. These techniques have the advantage of improved throughput because space charge effects are reduced but have the disadvantage of the complexities associated with the imaging optics and the imaging detectors, and have not been widely used.
The throughput advantages of pattern projection in electron beam lithography have been established in simulation and experiment. See, for example, J. E. Schneider et al, "Semiconductor on Glass Photocathodes as High-Performance Sources for Parallel Electron Beam Lithography", J. Vac. Sci. Technol. B, Vol. 14, No. 6, Nov./Dec. 1996, pp. 3782-3786 and A. W. Baum et al, "Semiconductor on Glass Photocathodes for High Throughput Maskiess Electron Beam Lithography", 41st Electron. Ion and Photon Beam and Nanolithography Conference, 1997.
Accordingly, there is a need for improved methods and apparatus for electron beam microscopy, wherein high resolution and high speed are achieved simultaneously, while minimizing added complexity.